The traditional image forming apparatus is provided with a motor, an image forming apparatus driving member and a process cartridge, wherein the process cartridge is detachably mountable to the image forming apparatus; and a photosensitive drum is arranged on the process cartridge. During the operation of the image forming apparatus, the driving force is generated by the motor and transmitted to the process cartridge via the image forming apparatus driving member, so that the photosensitive drum on the process cartridge is driven to rotate.
As illustrated in FIGS. 1 and 2, a driving force transmission mechanism is fixedly disposed at one end of the photosensitive drum 7 and comprises a drum shaft 17; a distorting protrusion 17a is disposed on the drum shaft 17 and has an end portion 17a1; the rotation center of the drum shaft 17 is coincident with the rotation center of the photosensitive drum 7; and the image forming apparatus driving member 18 is provided with a distorting groove 18a which has an bottom surface 18a2.
During the operation of the image forming apparatus, the image forming apparatus driving member 18 receives the rotating driving force from the motor; the drum shaft 17 is engaged with the image forming apparatus driving member 18; and the rotating driving force is transmitted to the drum shaft 17 via the image forming apparatus driving member 18 and finally drives the photosensitive drum 7 to rotate. When the drum shaft 17 is engaged with the image forming apparatus driving member 18, the distorting protrusion 17a on the drum shaft 17 is interposed into the distorting groove 18a on the image forming apparatus driving member 18; the end portion 17a1 is over against the bottom surface 18a2; and the rotating driving force from the image forming apparatus driving member 18 is transmitted to the drum shaft 17 via the engagement between the distorting protrusion 17a and the groove 18a. 
FIGS. 3 and 4 are cross-section diagrams respectively illustrating the state when the distorting protrusion 17a and the distorting groove 18a do not rotate and rotate. As illustrated in the figures, cross-sections of the distorting protrusion 17a and the distorting groove 18a are both triangles (such as equilateral triangles), and the dimension of the triangular protrusion 17a is less than that of the triangular groove 18a. As illustrated in FIG. 3, when the protrusion 17a is interposed into the groove 18a and does not rotate along with the groove 18a, a rotational axis X1 of the drum shaft on the photosensitive drum is not coincident with a rotational axis X2 of the image forming apparatus driving member. As illustrated in FIG. 4, when the protrusion 17a is engaged with the groove 18a and rotates along with the groove 18a, three vertex angles 17a2 of the triangular protrusion 17a are engaged with three edges of the triangle of the groove 18a, and the driving force is transmitted to the protrusion 17a from the groove 18a. Herein, the rotational axis X1 of the drum shaft on the photosensitive drum is coincident with the rotational axis X2 of the image forming apparatus driving member, so that stable transmission between the distorting protrusion 17a and the distorting groove 18a during the operation can be guaranteed. In the figure, RO refers to the rotating circle diameter of the three vertex angles 17a2 of the protrusion 17a; R1 refers to the inscribed circle diameter of the triangular groove 18a; and R2 refers to the rotating circle diameter of three vertex angles of the groove 18a. In order to achieve the driving force transmission between the protrusion 17a and the image forming apparatus driving member 18a, the RO, the R1 and the R2 must meet the following condition: R1<R0<R2.
FIG. 5 is another embodiment of the prior art. In the embodiment, both the distorting protrusion 17a and the distorting groove 18a are quadrangles (such as regular quadrangles), and the quadrilateral protrusion 17a and the quadrilateral groove 18a are engaged with each other and used for driving force transmission.
The image forming apparatus driving member in the prior art can also adopt the means as illustrated in FIG. 6. As illustrated in FIG. 6, a distorting groove 28a is disposed at one end of the image forming apparatus driving member 28 and provided with a bottom surface 28a1 and a supporting protrusion 28a2 which is disposed at the center of the triangular distorting groove 28a (the rotation center of the supporting column protrusion 28a2 is coincident with a rotational axis X2 of the image forming apparatus driving member); the height of the supporting protrusion 28a2 is basically the same with the depth of the groove 28a; and said supporting column can be conical.
The photosensitive drum provided with the driving force transmission mechanism is generally applied to a process cartridge for the traditional image forming apparatus. Said process cartridge at least comprises the photosensitive drum which is used for forming an electrostatic latent image and provided with the driving force transmission mechanism, a developer which is used for developing the electrostatic latent image, and a developing roller which is used for transmitting the developer to the photosensitive drum. When said process cartridge is mounted on the image forming apparatus, the rotating driving force from the motor on the image forming apparatus is received by said image forming apparatus driving member, so that the photosensitive drum and the developing roller are driven to rotate.
The driving force transmission mechanism in the prior art has the disadvantages that:
1. When the distorting protrusion is engaged with the distorting groove, the distorting angle of distorting surfaces on the protrusion and the groove is required to have high precision. When the distorting angle is inconsistent due to the manufacturing precision of the distorting surfaces on the protrusion and the groove, the distorting surfaces on the protrusion and the groove make point-to-surface contact, and one distorting surface may be deformed during the engagement between the protrusion and the groove, so that the rotation axis X1 of the drum shaft on the photosensitive drum cannot be coincident with the rotational axis X2 of the image forming apparatus driving member, and thus stable driving force transmission can be affected. In order to solve said problem, the manufacturing precision of the distorting surfaces of the protrusion and the groove is required to be high, so that the manufacturing cost can be increased and the problems such as difficult manufacture can be caused.
2. Polygonal shapes of the protrusion and the groove are difficult to process, and the manufacturing precision of the protrusion and the groove is required to be high. With an equilateral triangle as an example, the precision of the equilateral triangle protrusion and the equilateral triangle groove is required to be high, so that the precision at the triangular center can be guaranteed. Or else, the rotational axis X1 of the drum shaft on the photosensitive drum may not be coincident with the rotational axis X2 of the image forming apparatus driving member during the engagement between the protrusion and the groove, and then instable transmission can be caused. In addition, during the engagement between the protrusion and the groove, vertex angles of the triangle on the protrusion tend to be deformed after being stressed due to the requirement of driving force transmission and tend to be worn or damaged after long-term operation. Moreover, as the three vertex angles of the triangle also have the functions of stress rotation and supporting and positioning during the operation, the phenomenon that the centers X1 and X2 are not coincident with each other tends to occur during the engagement between the worn or damaged triangle and the groove, and thus stable transmission can be affected. Therefore, in order to guarantee the transmission precision and stableness, the hardness and wear resistance of materials of the triangular protrusion are required to be high. Meanwhile, as the positions at which the edges of the triangular groove make contact with the three vertex angles of the protrusion also tend to be damaged or worn during the operation, the hardness and wear resistance of the triangular groove are also required to be high.